A number of methods are presently used for the production of magnesium but only the thermal processes are of concern in this specification. One of the earlier processes is the Pidgeon process in which calcined dolomite and silicon, usually in the form of ferrosilicon are briquetted then charged into tubular, steel retorts and are then reacted. The energy required for the reaction is supplied externally of the tubular retorts. Temperatures of about 1450.degree. K. and pressures of below 14 Pa are commonly employed. The reaction is a solid state reaction with magnesium vapour being the product. The Pidgeon process suffers the disadvantage of low production capacity per unit and high maintenance costs.
Another of the earlier thermal processes to be developed is the carbothermic process. This process is based on the reaction of magnesium oxide with carbon to produce magnesium vapour. This process generally requires quenching of the magnesium vapour which produces magnesium powder and subsequent powder handling problems. South African Pat. No. 84/9885 describes an improvement to this process.
One of the later developments is the Magnetherm process. In this process, calcined dolomite is reacted with ferrosilicon, in the presence of alumina, in a submerged-arc reactor. The reaction with the ferrosilicon takes place in the molten state at or near the slag surface, which is generally above the thermal energy source which is generated by a submerged electrode. Thermal energy reaches the reaction zone by convection and conduction. The temperature within the reactor is normally in the region of 1820.degree. K. while pressures are normally below 4 kPa. This process suffers the disadvantage that the thermal energy source is below the reaction zone and, in consequence, slagging agents such as alumina or bauxite flux are preferably introduced to reduce slag liquidus temperature and viscosity, with resultant increased turbulence, mixing and heat transfer while staying within the constraints of sufficient electrical resistivity of slag and sufficient ferrosilicon holdup in the slag. Operation under vacuum increases leakage problems and dictates batch operation in practice. South African Pat. No. 84/7540 describes such a process as applied to reject slags of other pyrometallurgical processes.
It is accordingly an object of this invention to provide a process for the continuous production of magnesium which alleviates at least some of the problems of a process such as the Magnetherm process.